The answer is 1836 nucleotide bases.
It is known that three nucleotide bases in mRNA are called codon and that each codon codes for the specific amino acid. For 3 nucleotide bases in mRNA, there is 1 amino acid in a protein. So, there are 3 times more nucleotide bases in mRNA than amino acids in the protein. This can be represented mathematically:
3 nucleotide bases : 1 amino acid = x nucleotide bases : 612 amino acids
3 nucleotide bases x 612 amino acids : 1 amino acids = x nucleotide bases
x nucleotide bases = 612 x 3 nucleotide bases
x nucleotide bases = 1836 nucleotide bases.
So, mRNA long 1836 nucleotide bases will give a protein with 612 amino acids.